Excitable tissue control (ETC) devices are devices which modulate the activity of excitable tissues by application of non-excitatory electrical field signals to the excitable tissue through suitable electrodes in contact with the tissue. For example, ETC devices may be used, inter a/ia, to increase or decrease the contractility of cardiac muscle in vitro, in vivo and in situ., as disclosed in detail in PCT application PCT/IL97/00012 (International Publication Number WO 97/25098) to Ben-Haim et al., titled "ELECTRICAL MUSCLE CONTROLLER", incorporated herein by reference. Other methods and applications of ETC devices are disclosed in PCT application PCT/IL97/00231 (International Publication Number WO 98/10828) titled "APPARATUS AND METHOD FOR CONTROLLING THE CONTRACTILITY OF MUSCLES" to Ben Haim et al., incorporated herein by reference, PCT application PCT/IL97/00232 (International Publication Number WO 98/10829) titled "DRUG-DEVICE COMBINATION FOR CONTROLLING THE CONTRACTILITY OF MUSCLES" to Ben Haim et al., incorporated herein by reference and PCT application PCT/IL97/00233 (International Publication Number WO 98/10830) titled "FENCING OF CARDIAC MUSCLES" to Ben Haim et al., incorporated herein by reference, PCT application PCT/IL97/00235 (International Publications Number WO 98/10831) to Ben Haim et al., titled "CARDIAC OUTPUT CONTROLLER", incorporated herein by reference.
Further applications of the ETC including devices combining cardiac pacing and cardiac contractility modulation are disclosed in PCT Application, International Publication No. WO 98/10832, titled "CARDIAC OUTPUT ENHANCED PACEMAKER" to Ben Haim et al., co-assigned to the assignee of the present application. Such ETC devices function by applying to selected cardiac segments non-excitatory electrical signals of suitable amplitude and waveform, appropriately timed with respect to the heart's intrinsic electrical activity or with respect to paced cardiac electrical activity. The contraction of the selected segments can be modulated to increase or decrease the stroke volume of the heart. The timing of the ETC signals must be carefully controlled since application of the ETC signal to the myocardium at inappropriate times may be arrhythmogenic. The ETC signal must therefore be applied to the selected cardiac segment within a defined time interval during which the selected cardiac segment will not be stimulated by the ETC signal.
As disclosed in International Publication No. WO 98/10832, the ETC signal may be timed relative to a trigger signal which is also used as a pacing trigger, or may be timed relative to locally sensed depolarizing electrogram signals.
Timing of the delivery of ETC signals relative to the time of detection of locally sensed electrogram signals may present certain practical problems. For example, triggering of the ETC signal by any locally detected depolarizing signals irrespective of the time of detection of the depolarizing signal within the cardiac beat cycle, may increase the probability of spurious detection of noise signals or of ectopic beats such as premature ventricular contractions (PVCs) or the like, which may lead to delivery of improperly timed and potentially arrhythmogenic ETC signals. It is therefore desirable to have a method for determining proper timing of the delivery of ETC signals without unduly increasing the probability of delivering an improperly timed ETC signal caused by spurious noise detection or by detection of ectopic beats.